1. Field of the Invention
This invention relates generally to the field of energy harvesters, and more specifically to micromechanical energy harvesters.
2. Description of Related Art
Decades of research and development have been conducted in an attempt to harvest, i.e. collect, energy that exists in various forms in our environment. Solar cell technology is one of the more prevalent and well-known forms of energy harvesters, collecting solar energy and converting that energy into heat, electricity, or both. Another prevalent, but perhaps less known, energy harvester is the water turbine used in dam electric power plants. Moving water from a river or lake rotates a water turbine to produce electricity. Geothermal heating units installed in houses use a relatively new type of energy harvester. The geothermal heating unit recirculates a heat medium through a warmer area found deep beneath the surface layer.
Readily-available, low-frequency mechanical energy, such as the 1-2 Hz vibration source from walking, can be transformed into useful power for portable and wireless microsystems. Conventional energy harvesters traditionally operate as a single-degree-of-freedom (“single-DOF”) system to harvest environmental energy, such as vibrations, which can and do occur across multiple-degrees-of-freedom (“multi-DOF”). They are typically further limited by operating within a narrow frequency band about the resonance frequency of the single-DOF system. The efficiency of mechanical energy harvesters usually increase with frequency, but because the amount of useful environmental energy decreases with frequency, conventional energy harvesters sometimes use “upconversion” techniques to convert the relatively lower environmental ambient energy frequency to a more efficient, higher energy output frequency.